Simple Valve Preamplifier

[gingertube]

Theoretical maximum with no load is as you say 339V.

With losses thru 2 transformers and with a load it will probably be closer to 300V which is close enough to 260V for the circuit to work without modification. You can add a dropping resistor and an extra filter capacitor into the High Voltage line to reduce that if you are at all concerned. Or alternatively can can put dropping resistors into each of the AC High Voltage wires before the rectifier which will achieve the same thing.

BTW don't forget basic safety - extra fuse in the feed to the second transformer is a GOOD idea. You were already going to fuse the mains side of the first transforemer - right?

Cheers,
Ian

[David170]

Hi there,
I built one of those preamps a while back, I tried recording through my Mbox and it worked fine for guitar, though I think its a bit too accurate and clean perhaps for getting good gat tone. But I reckon it could be ok as a mic preamp. My question is, if I built this phantom power supply [ http://www.tangible-technology.com/p...om_frying.html ] , could I hook em up directly and record with a condenser? I've scoured the forums and apparently might need some kind of transformer. Maybe more.. If you could help me out on this I'd be super appreciative, coz, really, I have no idea.

Thanks

Dave

[Eli Duttman]

Originally posted by gingertube
Theoretical maximum with no load is as you say 339V.

With losses thru 2 transformers and with a load it will probably be closer to 300V which is close enough to 260V for the circuit to work without modification. You can add a dropping resistor and an extra filter capacitor into the High Voltage line to reduce that if you are at all concerned. Or alternatively can can put dropping resistors into each of the AC High Voltage wires before the rectifier which will achieve the same thing.

BTW don't forget basic safety - extra fuse in the feed to the second transformer is a GOOD idea. You were already going to fuse the mains side of the first transforemer - right?

Cheers,
Ian

Ian's suggestion can be extrapolated to use CLC filtration, with a smallish (say 1.5 muF.) cap. in the 1st position. The 30 H./40 mA. Hammond 157G will do very nicely for this project and have further use, in the future.

Use UF4007 diodes in the rectifier bridge. While only slightly more expensive than 1N4007s, UF4007s are MUCH less noisy.

[Gordy]

Originally posted by David170

But I reckon it could be ok as a mic preamp. My question is, if I built this phantom power supply...

...coz, really, I have no idea.

Thanks

Dave

Cue the background reading...

http://www.prodigy-pro.com/forum/viewtopic.php?t=16909

D, it might take a few hours to chew through it, but there is a lot of decent info there...

[MartyM]

I built that 12AX7 preamp and even ordered the PCBs from RCS Radio where you guys live (Australia). It's my first tube project, and overcoming the challenges presented by use of tubes in a vehicle was fairly easy using this design, with some additional circuits for vehicle-specific issues (ground loop isolation, heating timer, etc.).

I enjoyed the preamp, as long as I wasn't using an amplifer with too many preamp stages or "processing". That took away the valve sound almost entirely.

But I had problems with high-pitched noise with the transformer. Otherwise the SMPS works pretty well!

"The problem with this kit is that the switch mode PSU is noisy and you have to be nearly one metre away not to hear it when your room is quiet."

I knew it!! I was sure I wasn't the only one with this problem!! I emailed the editor @ Silicon Chip and he assured me that "that wasn't a problem with this type of design".

Uh-huh, sure. I have the exact same core, bobbin, and winding pattern...?

I will say this though: the quality of the PCBs from RCS was not fantastic [copper traces appear to be 'wrinkled'] and I had to modify slightly (drilled holes & used 9-pin ceramic sockets from ebay, added thru-hole RCA jacks, etc.).

Overall in my opinion a good starting point. Got them working right away.. ..just my $.02

[MartyM]

To everyone noting the noise SMPS for the Silicon Chip 12AX7 preamplifier:

I have been struggling with the noise issue for some time, and discovered after reading the TL494 data sheet (along with checking the waveforms) that the "DTC" pin (pin #4) appears to be missing a reference voltage, hence keeping the PWM function adjustment from working.

Also caused problems on startup when connected to another power supply for testing.

My transformer was horribly noisy until I reduced the duty cycle (it was about 90% until that point).

I simply added a 50K pot across the 10uF [between pins #2 & #4) and now it's quiet! I had to tinker with the adjustments, and I think my duty cycle is around 65%-70% now.

SILENCE IS GOLDEN!

I knew something wasn't right.....